GtkTargetEntry has been in gtk+ since 2.10
Remove all calls to gtk_dialog_set_has_separator
Drop support for gtk+ < 2.24
Always use g_spawn_command_line_sync
Use gdk_display* functions for both gtk+2 and gtk+3

The following functions are available in version 2 and 3.
gdk_display_get_default
GDK_DISPLAY_XDISPLAY
gdk_display_get_name
gdk_display_get_default